1. Field of the Invention
The invention relates to a polyethylene terephthalate resin composition. More particularly, the invention relates to a polyethylene terephthalate resin composition of a high crystallinity and excellent in moldability and in dimensional stability.
2. Description of the Prior Art
Polyethylene terephthalate resins have excellent properties, such as mechanical properties, chemical resistance, electrical properties and heat resistance, and are widely utilized for electrical insulating materials, automobile parts and the like. These properties of polyethylene terephthalate resins can be improved by the addition of various additives, such as a fiber reinforcement, e.g. glass fiber or carbon fiber, a function-imparting agent, e.g. flame retarder, and the like, and thus, the use of polyethylene terephthalate resins has increasingly been broadened.
Polyethylene terephthalate resins belong to crystalline resins and are characterized in that their properties largely depend upon the crystallinity thereof. Therefore, it is necessary to highten their crystallinity in order to improve the dimensional stability and deflection temperature thereof. Thus, there has been proposed various means for accelerating the crystallization of polyethylene terephthalate resins. For example, there have been proposed a method in which a molded article obtained in a partially crystallized state is subjected to post-heat treatment to further the crystallization process and a method in which such a molded article is dipped into a liquid capable of enhancing the crystallinity. However, these methods undesirably necessitate some post-treatments and have a further drawback in that the molded article may be deformed upon the post-treatment.
Further, there has been known a method in which, in order to accelerate the crystallization of a resin at the time of the fabrication, the molten resin is injected into a mold heated to a high temperature and then hardened within the mold. In this method, the use of a heating medium is necessary to maintain the mold at a high temperature, which is undesirable from the point of view of safety in the molding operation, and further, a long molding cycle time is required.
Recently, nucleating agents have been used for shortening the molding cycle time. Although such a method can afford a high crystallinity in a fairly short period of time, there is still room for improvement. For example, even in the case where a nucleating agent is added to a polyethylene terephthalate resin, it is necessary to use a mold of a high temperature, such as 130.degree. to 140.degree. C. In British Pat. No. 2,015,014, it is described that the use of a crystal growing agent in combination with a crystal nucleating agent can successfully lower the mold temperature to a fair extent. However, this method also leaves room for improvement in that the proposed crystal growing agent used is necessary in a fairly large amount and, thus, the properties inherent to the polyethylene terephthalate resin may deleteriously be affected.